1. Field of the Invention
The present invention is directed to a method and apparatus for reducing line artifacts in a CT image, wherein the CT image is produced by scanning with an X-ray source which is rotatable around an examination subject, with X-rays from the X-ray source, after being attenuated by an examination subject, being incident on a detector system.
2. Description of the Prior Art
CT devices are known which have an X-ray source, e.g. an X-ray tube, which direct a collimated, pyramid-shaped X-ray bundle through the examination subject, e.g. a patient, onto a detector system that is composed of a number of detector channels. Each detector channel has at least one detector element and one associated electronic element for reading out and amplifying the signal that is generated in the detector element as a result of the incident radiation. A number of detector elements can be allocated to one electronic element. The X-ray source and, depending on the construction of the CT device, the detector system as well are attached to a gantry that rotates around the examination subject. A support device for the examination subject can be displaced along the system axis relative to the gantry. The position along the system axis at which the X-ray bundle penetrates the examination subject, and the angle, under which the X-ray bundle penetrates the examination subject, are continuously modified as a result of the displacement and the rotation of the gantry. Each detector element of the detector system struck by the radiation produces a signal representing a measure of the overall transparency of the examination subject for the radiation proceeding from the X-ray source to the detector system. The set of output signals of the detector element, of the detector system, which is acquired for a specific position of the X-ray source, is referred to as a projection. A scan is composed of a set of projections, which are acquired at different positions of the gantry and/or at different positions of the support device. The CT device picks up a number of projections during a scan in order to be able to construct a two-dimensional tomogram of a slice of the examination subject. A number of slices can be picked up at the same time by a detector system that is composed of an array having a number of rows and columns of detector elements. Such planar-like detector systems, however, frequently contain detector channels which do not supply proper data. It may be that detector system contains faulty detector channels already after the production process, for example as a result of defects in fabrication caused by the high integration density of the electronic elements. Defects of individual detector channels also may arise during the operation of the CT device. Such defects cause circular structures in the acquired CT images, these circular structures being referred to as circle artifacts. Techniques referred to as xe2x80x9cring-balancingxe2x80x9d methods are known from the literature for the purpose of attempting to correct or prevent such artifacts in CT images. Such methods are disclosed in U.S. Pat. No. 4,670,840 and in German OS 198 35 451 (corresponding to U.S. Pat. No. 6,047,039), for example.
A disadvantage of such known methods is that they insufficiently eliminate artifacts which arise in a CT device having a defective detector channel.
An object of the present invention is to provide a method for reducing artifacts in a CT image, so that the obtainable image quality is improved in a CT device having at least one defective detector channel. It is also an object of the invention to provide a CT device for implementing the method.
The above object is achieved in accordance with the principles of the present invention in a method for reducing line artifacts in a CT image D1, as well as in an apparatus for implementing the method, wherein the image has been subjected to interpolated filtering for preventing or correcting faulty values of picture elements represented in a circle K1, and wherein the following steps are implemented. The picture elements of the CT image D1 are subjected to a median filtering, orthogonal to the straight line extending through the respective picture element and the center of the circle K1, for producing an image M1. A difference value image F1 is generated by subtracting the image M1 from the CT image D1. Two resulting images G11 and G21 are produced by filtering the picture elements of the difference value image F1 in the respective directions of tangents t11 and t21 to the circle K1 extending through the respective picture element. Filtering is conducted along t11 to produce the resulting image G11 and is conducted along t21 for producing the resulting image G21. A correction image D2 is then obtained by subtracting both of the resulting images G11 and G21 from the image D1.
Defective detector channels of a detector system lead to faulty values for picture elements, which appear as a circle in a CT image acquired by the detector system. Such image errors therefore are referred to as circle artifacts. Defective detector channels are not only ones that fail to supply an output signal as a result of the defect, but also are channels with a measuring accuracy that exceeds a specific tolerance value. Various methods are known for correcting or preventing circle artifacts, as noted above. These methods are carried out on the measurement data or on the image data and are primarily based on interpolated filtering, and achieve a noticeable weakening of the circle artifacts in the acquired CT images. Such known methods have the disadvantage, however, that line-like image errors frequently arise in the resulting CT images after such a method has been implemented. These image errors are referred to as line artifacts. They increasingly occur in association with large signal unsteadiness caused by high-contrast areas of an examination subject. The inventive method is particularly advantageous for eliminating such line artifacts, which occur after circle artifacts have been eliminated and which appear as tangents to the circles in the CT image. The size and position data of the circles of the circle artifacts caused by the defective detector channels are assumed to be known. It is sufficient to know the position of the circle center, which is the same for all circles, and the radii of the circles.
The elimination of the line artifacts is initially described for the case of a single defective detector channel. The faulty values of picture elements caused by the defective detector channel are situated on a circle K1. These faulty values are corrected by a known ring-balancing method on the measuring data or on the image data. Line artifacts, which appear as tangents to the circle K1, arise in the resulting CT image D1. For producing an image M1, a median filtering is carried out for each picture element of the CT image D1 situated outside of the circle K1, orthogonally to the straight line extending through the respective picture element and the center of the circle K1. The width of the median filtering can be modifiable. This width should be selected wider than the expected line width of the line artifacts. A median filter of the width 5 has proven to be beneficial. The sampling distance A1 must be selected dependent on the convolution kernel used for the reconstruction. Ideally, the image M1 no longer contains line artifacts.
A difference value image F1=D1xe2x88x92M1 is generated by subtracting the image M1 from the CT image D1. For producing two resulting images G11 and G21, a filtering is carried out in each picture element of the difference value image F1 in the respective directions of the tangents t11 and t21 to the circle K1 extending through the picture element in question, i.e., the filtering is carried out along t11 for the resulting image G11 and the filtering is carried out along t21 for the resulting image G21. This step is to eliminate, for the most part, the image pixel noise difference value in the image F1 for the most part. Furthermore, the line artifacts that are present in the error image F1 are emphasized in the result images. It is necessary to calculate two result images G11 and G21, since there are two possible tangent directions to the circle K1 per picture element. The thus-determined resulting images G11 and G21 are subtracted from D1 and the correction image D2 is obtained, which ideally no longer contains line artifacts generated by the defective channel.
The correction image D2 serves as basis image for correcting the line artifacts caused by other detector channels if the detector system contains further defective detector channels. In general, the correction image D1 serves as a basis image for correcting line artifacts caused by the i-th defective detector channel. Since the image M1 has been calculated in relation to the center of the circle that is valid for all circles ki, the difference value image F1 can still be used in the following. For producing the result images G1i and G2i, a filtering is carried out in each picture element of the difference value image F1 situated outside of the circle Ki in the direction of the tangents D1i and D2i to the circle Ki, i.e., the filtering is carried out along D1i for the resulting image G1i and the filtering is carried out along D2i for the resulting image G2i. The correction image Di+1=Dixe2x88x92G1ixe2x88x92G2i is determined by subtracting the resulting images G1i and G2i from Di. This procedure is repeated until the line artifacts of all defective detector channels are eliminated.
The above-described method is relatively time-consuming as a result of the serial processing of the line artifacts caused by the defective detector channels. Parallel processing of the image errors caused by the different defective detector channels is preferable for processing the data faster. Accordingly, a further embodiment of the inventive method proceeds as described above until the calculation of the difference value image F1. The resulting images G1i and G2i are calculated in this manner for all defective detector channels. In contrast to the initially described embodiment, different correction images are not consecutively determined, which respectively serve as an image for calculating the next correction images, but instead all resulting images G1i and G2i are subtracted from D1. The correction image D2 results. The defective detector channels should have a minimum distance (spacing) of ten detector channels as a condition for this parallel processing.
The aforementioned median filtering for producing an image M1, in each picture element of the initial CT image, takes place orthogonally to the straight line extending through the picture element in question and the center of the circle Ki. This calculation represents an approximation, and it is an advantage of the inventive method that it must be carried out only once for each picture element even given a number of defective detector channels. Better results are obtained, however, by the median filtering orthogonal to the tangents to the circles Ki. This makes it necessary to carry out the pixel-oriented median filtering anew for each defective detector channel, with an associated high computing outlay. Given a single defective detector channel and the faulty values of picture elements on a circle K1 caused as a result thereof, the correction includes the following steps:
carrying out an interpolated filtering of the artifact-containing CT image, thereby leading to the CT image D1, in order to avoid or to correct faulty values of picture elements on the circle K1,
carrying out a median filtering in each picture element of the CT image D1 situated outside of the circle K1, orthogonal to the tangents to the circle K1 extending through the respective picture element, for producing an image M1,
generating a difference value image F1=D1xe2x88x92M1 by subtracting the image M1 from the CT image D1,
carrying out a filtering in each picture element of the difference value image F1 situated outside of the circle K1 in the direction of the tangents t11 and t21 to the circle K1 extending through the respective picture element in order to produce two resulting images G11 and G21, with the filtering being carried out along t11 for the resulting image G11 and the filtering is carried out along t21 for the resulting image G21.
determining the correction image D2=D1xe2x88x92G11xe2x88x92G21 by subtracting the resulting images G11 and G21 are from D1.
If a number of defective detector channels are present, the following applies for processing the i-th defective detector channel:
a median filtering is carried out in each picture element of the CT image Di, orthogonal to the tangents to the circle Ki extending through the respective picture element, for producing an image Mi,
a difference value image Fi=Dixe2x88x92Mi is produced by subtracting the image Mi from the correction image Di,
a filtering is carried out in each picture element of the difference value image F1 situated outside of the circle K1 in the direction of the tangents t1i and t2i to the circle Ki extending through the respective picture element in order to produce two resulting images G1i and G2i, with the filtering being carried out along t1i for the resulting image G1i and the filtering being carried out along t2i for the resulting image G2i.
the correction image Di+1=Dixe2x88x92G1ixe2x88x92G2i is determined by subtracting the resulting images G1i and G2i from Di.
These steps are repeated until all image errors caused by the different defective detector channels are processed.
In a further version of the invention, a running averaging serves as the filtering for producing the resulting images G1i and G2i. In another version of the invention, a sum operator is implemented as the filtering for producing the resulting images G1i and G2i. A combination of these two versions also can be employed, wherein a running averaging and a sum operator are simultaneously employed for filtering.
At the beginning of each of the aforementioned embodiments, the values of the CT image D1 are preferably limited to a range 1000 HU (H2O)xc2x1xcex94. This limitation is expedient for examining soft-tissue parts, in which disturbances caused by defective detector channels are particularly apparent in the image.
The intensity of the noise signal in the resulting images G1i and G2i is dependent on the filter width of the running averaging or of the sum operator. This noise signal influences the correction images Di+1 and therefore can lead to undesired noise structures in the resulting image in the case of a number of defective detector channels. A high filter width is required as a result. The intensity of a line artifact varies, however, so that a limitation of the filter width of the running averaging or of the sum operator is required.
The summation of the HU values in G1i in the tangent direction D1i or in G2i in the tangent direction D2i represents an additional measure for suppressing noise. This corresponds to the calculation of the radon values in these tangent directions. This operation of the image processing is referred to as HUG transformation. On the basis of a threshold criterion, a noise signal can be principally differentiated from artifact structures and can be eliminated. This threshold must be suitably selected, however, so that low-contrast lines in G1i and G2i, that are actually a part of the diagnostically relevant image context, are not erroneously eliminated.